Silicon on sapphire (SOS) structures in which a thin film of silicon is attached to an insulating sapphire substrate have been made in the past. Microelectronic devices such as transistors, diodes, etc. may be fabricated in the silicon film with the sapphire substrate providing insulation between the devices.
The SOS structures have been fabricated using chemical vapor deposition (CVD) techniques to adhere a crystalline epitaxial film of silicon (Si) onto a sapphire substrate. A problem which has occurred is that the silicon film will contain a high concentration of crystal defects such as stacking faults or twins. The highest concentration of defects occurs at the interface between the sapphire substrate and the silicon film. The lowest concentration of defects occurs at the outward surface of the silicon film, away from the sapphire substrate. The silicon film also experiences a built-in strain produced during the deposition of the silicon film on the sapphire substrate. The presence of defects and strain in the CVD epitaxial SOS is attributed to the 10% mismatch in lattice spacings and the 100% difference in thermal expansion coefficients of Si and sapphire.
Techniques using a surface-regrown (SRG) SOS process have been developed in order to reduce the effects of the defects. The techniques have involved implanting .sup.28 Si ions into the silicon film so as to amorphize all but the surface of the silicon film (i.e., the silicon film is made amorphous except for a thin surface layer which remains crystalline). The range of the implanted silicon atoms is chosen to be approximately equal to the thickness of the silicon film. The amorphized silicon film is then epitaxially recrystallized from the surface inwards, resulting in a reduction in the level of the planar lattice defects near the silicon-sapphire interface. However, the quality of the non-amorphized silicon surface region is not improved by this process. Such techniques result in positive shifts in threshold voltage levels for transistors fabricated in such surface-regrown (SRG) SOS, which are correlated with p-type doping due to aluminum (Al) penetration from the silicon-sapphire interface. This latter effect is believed to be inherent in this SRG SOS process, since it is only observed after both silicon (Si) implantation and recrystallization steps.